A high-definition detector has been developed in recent years. A larger number of X-ray detecting elements can be mounted in the same area on the high-definition detector, because a size of each X-ray detecting element is smaller than that of an X-ray detector of a normal X-ray CT apparatus. In an X-ray CT apparatus including such a high-definition detector (hereinafter, referred to as the high-definition X-ray CT apparatus), the number of X-ray detecting elements is larger than that of the normal X-ray CT apparatus, and hence data volume acquired by the high-definition X-ray CT apparatus is larger accordingly.
Consequently, it is conceivable to compress raw data acquired by the high-definition X-ray CT apparatus (hereinafter, referred to as the high-definition data). In a case of generating a reconstruction image with an image quality equivalent to that of an image acquired by the normal X-ray CT apparatus, it is necessary to restore the original high-definition data. In such a case, extra time is necessary for the restoration of the original high-definition data, and the time required for a reconstruction process is longer as a whole.